Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or grease it with non-stick cooking spray.
In a mixing bowl, combine the old-fashioned oats, all-purpose flour, brown sugar or coconut sugar, baking powder, baking soda, salt, and cinnamon. Stir until the dry ingredients are well combined.
In a separate bowl, whisk the egg. Add the melted coconut oil and vanilla extract to the bowl, and whisk again until the wet ingredients are well combined.
Mash the ripe bananas with a fork until smooth and add them to the bowl with the wet ingredients. Stir until everything is thoroughly mixed.
Gradually add the wet ingredients to the dry ingredients. Mix until just combined. Be careful not to overmix, as it can result in denser muffins.
Using a spoon or scoop, fill each muffin cup about three-fourths full with the batter.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the tin for a few minutes. Then transfer them to a wire rack to cool completely.